The invention relates to a process for operating a motor vehicle heating system in a motor vehicle coolant circuit which contains a motor vehicle engine, a motor vehicle heater with a heat exchanger and associated blower. The invention also relates to a fuel-operated heating device, especially a motor vehicle auxiliary heating device that is operable independent of the engine, and which is connected upstream from the motor vehicle heater, as well as a motor vehicle heating system with the units mentioned above.
From German Offenlegungsschrift 35 21 372, a heating system for motor vehicles is known which contains a heating device that is operated by fuel independent of the engine in a coolant circuit that has a motor vehicle heat exchanger and a motor vehicle heating blower. A temperature sensor in the heating circuit detects the temperature of the coolant and if it is lower than 50.degree. C., for example, the speed of a circulating pump in the heating circuit is lowered by a control device to accelerate the warming of the coolant as it passes through the heating device. Depending on the temperature of the coolant in the heating circuit, the speed of the circulating pump can be raised in steps or it can be continuously adjusted in an infinitely variable manner up to the nominal speed.
In this motor vehicle heater, a particularly significant reduction of the time which elapses between the turning on of the heating device and the releasing of heat by the heat exchanger into the passenger compartment is achieved, so that heating comfort in the motor vehicle is improved. With this heater, the heating of the passenger compartment is given priority, while the preheating of the motor vehicle engine connected in the coolant circuit is possible only by the residual heat remaining downstream from the motor vehicle heat exchanger. However, a homogeneous heating of the motor vehicle engine, which is particularly effective for favorable starting performance, cannot be attained this way.
From German Offenlegungsschrift 23 51 472, a process is known for rapid preheating of a water-cooled motor vehicle engine and for rapid heating of the interior space of the motor vehicle, as is an arrangement for carrying out the process. In this case, after the fuel operated heating device is turned on, the intake and exhaust side of this device is short-circuited to bring the coolant circulating in this short circuit rapidly to a temperature greater than 60.degree. C. After this temperature is reached, the heated coolant is then conveyed through the heat exchanger of the motor vehicle to heat up the interior space of the motor vehicle rapidly. After the heated coolant passes through the heat exchanger of the motor vehicle, the coolant goes back through the motor vehicle engine and to the fuel operated heating device.
Even in this process, the heating of the passenger compartment is given priority. As a result, with such a motor vehicle heating system, difficulties in starting the motor vehicle engine can also occur, and in addition, these difficulties in starting can depend on the size or the engine volume of the motor vehicle engine.